But teachers’ unions have raised concerns that members have been made more vulnerable to possible attacks since the Tory-led Coalition introduced powers in 2011 for teachers to search pupils for weapons.

Chris Keates, leader of the NASUWT teachers’ union, said that the increase in the number of teachers buying stab-proof vests “may be a consequence of this change and indicates ministers are failing to make schools safer places for staff and pupils”.

School shootings in the US are rife - there have been 39 incidents so far this year alone, resulting in the deaths of 12 people.

One of the worst shootings in recent times was the Sandy Hook elementary School massacre which left six members of staff and 20 children dead.

The Virginia Tech massacre of 2007 saw gunman Seung-Hui Cho shoot dead five teachers and 27 students before turning the gun on himself.

Teachers in England have also faced online death threats from pupils through abusive social media messages.

A survey released earlier this month showed that 21% of teachers had abusive or negative comments posted about them online - including one pupil who threatened to kill his teacher and invited fellow pupils to join in.